The invention relates to NDRO (nondestructive readout) core memories including at least two ferrite cores per memory bit, and more particularly to techniques for increasing the amplitudes of the sense voltages for each memory bit.
Most memory systems composed of ferrite cores have been DRO (destructive readout memories), wherein the read current forced through a core in order to induce a sense voltage on the sense-digit line destroys the stored state of that core. DRO core memories consequently usually include a restore cycle in which the sensed voltage is stored in a register and then rewritten back into the core. In certain applications wherein it is unacceptable to lose data in such a register, it is desirable to not be required to perform the restore cycle required by DRO memories. Thus, a market has developed for NDRO core memories, in which the read currents are small enough to avoid destroying the state of the memory bit being sensed. Unfortunately, use of smaller amplitude read currents results in reducing the amplitude of the sense voltages induced across the cores by the read currents. The sense voltages are already so small, typically 1 millivolt per core pair, that they are comparable to various kinds of electronic noise present within a large core array. (A plurality of "core pairs" can be used in each NDRO memory cell to boost the sense voltages, as is well-known by those skilled in the art.) The state-of-the-art for NDRO core memories is shown in U.S. Pat. Nos. 3,375,504 and 3,323,114. NDRO thin film magnetic memory systems have also been proposed, but have not proven as practical as NDRO core memories. The smaller sense voltages obtainable from NDRO core memories cause various problems in design of such memories, limiting memory speeds, maximum memory size, complexity and cost of sense circuits, etc. In some instances, it has been necessary to use four or six ferrite cores per memory bit in order to boost the sense voltages to adequate levels. This adds somewhat to the cost of memories of such NDRO core memories.
Despite the considerable demand for an improved NDRO core memory that provides larger sense voltages than has been heretofore achievable, those skilled in the art have been unable to appreciably improve the sense voltages of NDRO core memory systems.